This application claims the priority benefit of Taiwan application serial no. 90105284, filed on Mar. 7, 2001.
1. Field of the Invention
The invention relates in general to an on-line monitoring method. More particularly, this invention relates to a method of on-line monitoring the loss of silicon nitride.
2. Description of the Related Art
In the semiconductor fabrication process, how to reduce the fabrication cost and to enhance the device performance become on of the most important topics. To reduce the fabrication cost of the semiconductors, the number of devices fabricated on a wafer is increased. That is, the device integration on each wafer is increased. To increase the device integration, the dimension of devices has to be reduced. As a consequence, the dimensions of contact openings are reduced.
FIG. 1 shows a cross sectional view of a share contact opening. An isolation structure 102, a source/drain region 104 and a gate 106 are formed in a substrate 100. A silicon nitride spacer 108 is formed on both sidewalls of the gate 106. A dielectric layer 110 is then formed over the substrate 100. A share contact opening 112 is formed in the insulating layer 110 to expose a portion of the source/drain region 104 next to the gate 106 and a part of the spacer 108.
As the exposed part of the spacer 108 is easily corroded by the etchant used for forming the contact opening 112, the part of the spacer 108 is inevitably lost to result in the spacer 108a as shown in FIG. 1. The loss of the spacer 108 cause a leakage current between the isolation structure 102 formed under the gate 106 and adjacent to the source/drain region 104. As a consequence, the product electrical characteristics and yield are seriously affected.
An optical instrument is typically used to measure the loss of silicon nitride spacer. However, as the integration increases, the dimensions of the contact openings are often narrow to correctly measure the loss of silicon nitride spacer using the optical instrument.
The invention provides a method for monitoring loss of an etch stop layer on-line, applicable to monitor the loss of a first etch stop layer below a first insulating layer in a first contact opening after the first contact opening is formed in the first insulating layer over a device region and scribe line of a wafer. A dummy wafer is provided on which stacks in sequence a second etch stop layer and a second insulating layer. The second insulating layer is patterned by removing a portion of the second insulating layer, so that a monitoring opening that exposes the second etch stop layer and a second contact opening are formed in the second insulating layer. A first measuring step is performed to measure a first thickness loss and a second thickness loss from the second etch stop layer exposed respectively by the monitoring opening and the second contact opening on the dummy wafer. And a correlation is established from the first and second thickness losses. A second measuring step is performed to measure a third thickness loss from the first etch stop layer exposed by the monitoring opening on the wafer. The result is then compared with the correlation to deduce a fourth thickness loss from the first contact opening on the wafer.
Since the stack layers (comprising at least the second insulating layer and the second etch stop layer) formed on the dummy wafer are the same as those formed on the device wafer, the loss of second etch stop layer below the second insulating layer of the dummy wafer opening can easily be monitored on line after the monitoring opening and the second contact opening having the same pattern as the first contact opening are formed. A mathematical correlation is then established from the second thickness loss in the second contact opening and the first thickness loss in the first contact opening. A thickness loss from the first etch stop layer exposed by the monitoring opening on a scribe line of the device wafer is then monitored by an optical method, so as to deduce a thickness loss of the first insulating layer in the first contact opening over the device wafer. Therefore, the condition of the operation machine can be real time monitored and adjusted to ensure the product yield.
Both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.